Liquid detecting apparatus, liquid-amount detecting apparatus, liquid detecting method, and liquid-amount detecting method

ABSTRACT

A liquid-amount detecting apparatus detects the amount of a liquid in containers. The liquid-amount detecting apparatus includes a liquid detecting circuit and a determining unit. The liquid detecting circuit includes electrode units disposed so as to be in contact with the liquid in the containers, which becomes electrically conductive when in contact with the liquid, an impedance, and an AC-signal source. An AC signal not containing a DC component is input from the AC-signal source to the electrode units through the source impedance, and a signal representing the status of electric connection of the electrode units is output. Furthermore, based on the output signal, a binary signal representing the presence or absence of electrical connection of the electrode units is output. The determining unit determines the presence or absence of the liquid at the electrode units based on the binary signal output from the liquid detecting circuit.

The subject matter of application Ser. No. 10/625,120 is incorporatedherein by reference. The present application is a continuation of U.S.application Ser. No. 10/625,120, filed Jul. 23, 2003, now U.S. Pat. No.6,962,079 which claims priority to Japanese Patent Application No.JP2002-215848, filed Jul. 24, 2002. The present application claimspriority to these previously filed applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid detecting apparatus or aliquid-amount detecting apparatus for detecting a liquid or the amountof a liquid in a container. For example, the present invention relatesto an apparatus for detecting the remaining amount of ink in an ink tankof an ink-jet printer.

2. Description of the Related Art

In an ink-jet printer, ink is stored in an ink tank, and the ink istransferred from the ink tank to an ink discharging unit (head) throughan ink path, whereby droplets of ink is discharged. In the ink-jetprinter, the presence or absence of ink must be detected with arelatively high precision. A first reason for this is that it isdifficult to visually determine the remaining amount of ink from theexternal appearance of the ink tank.

As a second reason, if the ink is discharged to such an extent that theink is completely used up, ink that serves as “primer” becomesunavailable. That is, air enters the ink path, preventing the ink frombeing transferred to a nozzle. In that case, ink must be supplied againfrom the start to allow ink to be discharged, or the ink dischargingunit could be degraded. As a method of discharging ink in an ink-jetprinter, the thermal method is known, in which ink in an ink cell israpidly heated by a heating element to discharge droplets of ink. If theheating element generates heat even though ink is not present, theheating element could be damaged. Thus, discharging of ink (printing)must be stopped when the remaining amount of ink reaches a certainlevel.

Furthermore, as a third reason, when a large-sized print sheet is used,if the remaining amount of ink is not detected precisely, it is possiblethat ink is used up during printing and printing up to that time becomesvain.

From the viewpoints of safety, economy, etc., described above, it isnecessary to detect the remaining amount of ink accurately.

Known method of detecting the remaining amount of ink include (1)mechanical detection, (2) optical detection, (3) detection based onchange in electrical resistance, (4) detection based on change incapacitance, and (5) detection based on count of discharged amount.

Examples of (3) detection based on change in electrical resistanceinclude (1) Japanese Unexamined Patent Application Publication No.6-226990 (Patent Document 1), (2) Japanese Patent Publication No.2772015 (Patent Document 2), (3) Japanese Patent Publication No. 2798948(Patent Document 3), and (4) Japanese Unexamined Patent ApplicationPublication No. 11-179936 (Patent Document 4).

Of the examples of detection based on change in electrical resistance,according to the methods disclosed in Patent Documents 1 to 3, a pair ofelectrodes is provided in a liquid, and a current is fed to theelectrodes from a DC power source via a resistor having a high value ofresistance. The voltage applied to the pair of electrodes changesdepending on the presence or absence of the liquid between the pair ofelectrodes. According to the method disclosed in Patent Document 4, analternating current is used for detection of a liquid.

The related art described above, however, has had the followingproblems.

First, when a direct current flows through a liquid as in the artdisclosed in Patent Documents 1 to 3, an electrolysis occurs dependingon the type of the electrodes and the components of the liquid. Thus,the surfaces of the electrodes are likely to change, and metallic ionsare eluted into the liquid, possibly causing change in thecharacteristics of the liquid (ink). Furthermore, according to themethods that use a direct current, as will be described later inrelation to embodiments of the present invention, the impedance of thecircuit system tends to be high, which makes a detection at high speeddifficult.

According to the art disclosed in Patent Document 3, in order toovercome this drawback, the direction of a current that flows throughelectrodes is reversed at a cycle of measurement period. According tothis method, however, a direct current is used for measurement itself,and ions generated by the measurement with the direct current areeliminated by a flow of a DC current in the reverse direction for thesame length of time. Therefore, the speed of measurement is slow.

According to the art disclosed in Patent Document 4, since analternating current is used, the problem of the electrolysis does notoccur. However, liquid is detected in an analog manner, i.e., bydetecting change in the amount of the liquid based on change incapacitance. Thus, levels detected are unstable, and results ofdetection are not reliable.

SUMMARY OF THE INVENTION

Accordingly, a main objective of the present invention is to preventelectrolysis (ionization) of liquid from occurring and not to change incharacteristics of the liquid while allowing reliable detection.

The present invention, in one aspect thereof, provides a liquiddetecting apparatus for detecting a liquid contained in at least onecontainer, the liquid detecting apparatus including a liquid detectingcircuit including an electrode unit formed, at least, by a pair ofelectrodes that is to be disposed in contact partially with the liquidin the container, the pair of electrodes being electrically connected toeach other when the pair of electrodes is in contact with the liquid; animpedance; and an alternating-current signal source; wherein the liquiddetecting circuit inputs an alternating-current signal that does notcontain a direct-current component to the electrode unit via a certainvalue of impedance, outputs a signal representing status of electricalconnection between the pair of electrodes, and outputs a binary signalrepresenting the presence or absence of electrical connection betweenthe pair of electrodes based on the output signal; and a determiningunit for determining the presence or absence of the liquid at theelectrode unit based on the binary signal output from the liquiddetecting circuit.

The present invention, in another aspect thereof, provides aliquid-amount detecting apparatus for detecting the amount of a liquidcontained in at least one container, the liquid detecting apparatusincluding a liquid detecting circuit including an electrode unit formedby a pair of electrodes that is to be disposed in contact at leastpartially with the liquid in the container, the pair of electrodes beingelectrically connected to each other when the pair of electrodes is incontact with the liquid; an impedance; and an alternating-current signalsource; wherein the liquid detecting circuit inputs analternating-current signal that does not contain a direct-currentcomponent to the electrode unit through a certain value of impedance,outputs a signal representing status of electrical connection betweenthe pair of electrodes, and outputs a binary signal representing thepresence or absence of electrical connection between the pair ofelectrodes based on the output signal; and determining unit fordetermining the presence or absence of the liquid at the electrode unitbased on the binary signal output from the liquid detecting circuit.

The present invention, in another aspect thereof, provides a liquiddetecting method for detecting a liquid contained in at least onecontainer, wherein an alternating-current signal that does not contain adirect-current component is input from an alternating-current signalsource to an electrode unit via a certain value of impedance, theelectrode unit being formed by a pair of electrodes that is to bedisposed in contact at least partially with the liquid in the container,the pair of electrodes being electrically connected to each other whenthe pair of electrodes is in contact with the liquid, wherein a signalrepresenting status of electrical connection between the pair ofelectrodes is output, wherein a binary signal representing the presenceor absence of electrical connection between the pair of electrodes isoutput based on the output signal, and wherein the presence or absenceof the liquid at the electrode unit is determined based on the binarysignal.

The present invention, in another aspect thereof, provides aliquid-amount detecting method for detecting the amount of a liquidcontained in at least one container, wherein an alternating-currentsignal that does not contain a direct-current component is input from analternating-current signal source to an electrode unit through a certainvalue of impedance, the electrode unit being formed by a pair ofelectrodes that is to be disposed in contact at least partially with theliquid in the container, the pair of electrodes being electricallyconnected to each other when the pair of electrodes is in contact withthe liquid, wherein a signal representing status of electricalconnection between the pair of electrodes is output, wherein a binarysignal representing the presence or absence of electrical connectionbetween the pair of electrodes is output based on the output signal, andwherein the presence or absence of the liquid at the electrode unit isdetermined based on the binary signal.

According to the present invention, an alternating-current signal thatdoes not contain a direct-current component is input from thealternating-current signal source to the electrode unit through acertain value of impedance.

When the alternating-current signal is input to the electrode unit, asignal representing the status of electric connection between the pairof electrodes of the electrode unit is output, and a binary signalrepresenting the presence or absence of electric connection between thepair of electrodes is output based on the output signal. The presence orabsence of liquid at the electrode unit is determined based on thebinary signal.

Thus, since an alternating-current signal that does not contain adirect-current component is input to the electrode unit, a directcurrent does not flow through the liquid, avoiding change in thecharacteristics of the liquid. Furthermore, thanks to a low conductionimpedance, detection speed can be increased.

Furthermore, since the presence or absence of liquid is determined byoutputting a binary signal, digital processing is allowed, serving toimprove the reliability of detection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an equivalent impedance circuit;

FIG. 2 is a diagram showing the results of a simulation of theequivalent impedance circuit shown in FIG. 1;

FIGS. 3A and 3B are diagrams specifically showing the difference incircuit impedance due to difference between DC detection and ACdetection;

FIG. 4 is a diagram showing the construction of a liquid-amountdetecting apparatus according to an embodiment of the present invention;

FIG. 5 is a waveform chart for explaining a detecting operationaccording to a first embodiment;

FIG. 6 is a diagram showing a liquid detecting circuit in the firstembodiment;

FIG. 7 is a waveform chart showing a second embodiment of the presentinvention, and it corresponds to FIG. 5 for the first embodiment;

FIG. 8 is a diagram showing a third embodiment of the present invention,and it corresponds to FIG. 6 for the first embodiment;

FIG. 9 is a waveform chart relating to the circuit diagram shown in FIG.8, and it corresponds to FIG. 5 for the first embodiment;

FIG. 10 is a circuit diagram showing a fourth embodiment of the presentinvention, and it corresponds to FIG. 6 for the first embodiment; and

FIG. 11 is a waveform chart relating to the circuit diagram shown inFIG. 10, and it corresponds to FIG. 5 for the first embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, embodiments of the present invention will be described withreference to the accompanying drawings.

According to the present invention, a pair of electrodes is disposed ina liquid, and the presence or absence of the liquid is determined basedon a current that flows between the pair of electrodes. The current thatis used herein is an alternating current, not a direct current. Thereasons for this will be described below.

When the electrical resistance between electrodes that are in contactwith a liquid is measured, for example, by a circuit tester, theelectrical resistance does not depend much on the distance between theelectrodes, and the electrical resistance is large at first, and becomeslower as time elapses. This phenomenon can be explained by increase inions due to progress of electrolysis that is caused by the measurementitself with a direct current between the electrodes, although thephenomenon depends considerably on the material of the electrodes, theconditions of surface treatment, the area of surfaces in contact withthe liquid, the characteristics of the liquid, etc. Now, let thedistance between the electrodes be denoted as L and the cross sectionalarea of the electrodes through which the current flows be denoted as A.Then, L/A is known to be a constant value (=K) in a given container. Theresistance R of the liquid between the electrodes is R=K/k, where kdenotes the conductivity of the liquid.

Considering the impedance (Zx) between the electrodes based on what hasbeen described above, an equivalent impedance circuit shown in FIG. 1 isconsidered as appropriate. In FIG. 1, a resistor Rdc represents theresistance of the liquid as measured with a direct current over a shortperiod. A capacitor Cx represents the electrostatic capacitance of theliquid. A resistor Rac represents the resistance of the liquid asmeasured with an alternating current. Since values of measurement differbetween measurement with a direct current and measurement with analternating current, the capacitor Cx is connected in series with theresistor Rac.

FIG. 2 is a diagram showing the results of a calculation of theequivalent impedance circuit shown in FIG. 1. In the calculation, inkfor an ink-jet printer was chosen as the liquid.

In FIG. 2, the horizontal axis represents the frequency (Hz), and thevertical axis represents the output voltage (mV) between the electrodesof the equivalent circuit shown in FIG. 1.

Furthermore, in the circuit shown in FIG. 2, a signal source V1 is anAC-signal source, and a resistor R2 is a signal-source resistor.

As is apparent from FIG. 2, under the conditions of the liquid and theelectrodes in the calculation, at frequencies not higher than 100 Hz,the resistance as measured with an alternating current is several MΩ,which is substantially equivalent to the resistance as measured with adirect current; however, at frequencies from 100 Hz to 1 kHz, theresistance dramatically decreases (3 MΩ to 500 Ω), and even decreases toapproximately 1/10,000 depending on the conditions of the liquid and theelectrodes.

This indicates the following:

(1) In detecting a liquid using the electrical resistance (orconductivity) of the liquid as a switch, when a direct current is used,a high conduction resistance is inevitable in a conductive state (whenthe liquid is present between the electrodes); however, with analternating current having a frequency of several kHz or higher, theoverall impedance can be reduced by three or four orders of magnitude.

(2) According to the calculation described above, the resistance of theliquid as measured with an alternating current is a low/constant valueover a considerably wide band in frequency. Thus, when an AC signal thatfalls in this frequency band is applied to the switch from the signalsource via the series resistor, the switch exhibits a highopen/short-circuit ratio.

(3) It is possible to prevent an effect of stray capacitance and/orcross-talks, because the circuit impedance is low. In particular, thistendency is reinforced if an output value of detection is binary.

FIGS. 3A and 3B are diagrams specifically showing the difference incircuit impedance between detection with a direct current and that withan alternating current. FIG. 3A shows a model of detection with a directcurrent, and FIG. 3B shows a model of detection with an alternatingcurrent. In FIGS. 3A and 3B, V1 and V2 denote signal sources,respectively, and a resistor Rg is a signal-source resistor. Cs denotesa stray capacitance between the electrodes. S-Sw denotes an electrodeselecting switch, and W-Sw denotes a switch that operates based onconduction through the liquid.

The principal difference between DC detection and AC detection is thatonly one threshold (whether a certain level is exceeded or not) is usedin DC detection whereas two thresholds centered at 0 are usually used inAC detection.

In DC detection, it is required that a current that flows between theelectrodes be minimized in order to alleviate the problem of ionization.

For this reason, the values of the signal-source resistor Rg and theinter-electrode resistor Rdc must be very large, as in the DC detectionmodel shown in FIG. 3A. Thus, the effect of the stray capacitance Csattributable to wiring extending from the signal-source resistor Rg tothe electrodes and the electrodes themselves becomes larger.

In the example shown in FIG. 3, the value of the signal-source resistorRg differs by three orders of magnitude between DC detection and ACdetection. This difference leads to a difference in time that is takenbefore the conditions of the electrodes stabilize and measurement can bestarted.

For example, in the case of DC detection, assuming that the straycapacitance Cs is 5 pF, Tdc is a large value on the order of 50 μsec.Thus, a single detecting circuit suffices if the number of electrodes issmall (e.g., if a rough detection suffices or if the number ofcontainers to be monitored is small) or if a slow cycle of overalldetecting operation is acceptable.

However, for example, in the case of an ink-jet printer, the remainingamounts of ink of four to seven colors in different containers must bedetected at a high speed and with a high precision, and the distancesbetween the electrodes and the containers tend to be long. In that case,it is possible that a detection circuit is required for each color and asingle detecting circuit does not suffice, or the circuit configurationbecomes complex.

Furthermore, in DC detection, measurement of a peak value is critical inorder to check to what extent a voltage applied to the electrodes risesin a given time. Thus, a peak detector is usually used. In the peakdetection, in principle, a value detected must be held until the valueis output as valid data, and the value must be cleared before a nextmeasurement takes place. That is, in DC detection, in addition to a risetime, an extra time is taken in measurement due to an analog delay ofthe stray capacitance and clearing of a previous value of the peakdetector. Thus, the overall measurement takes a longer time.

On the other hand, in AC detection, since the original circuit impedanceis lowered by the conductivity of the liquid, the time taken untilconvergence to a peak value is much shorter than in the DC detection,and timing for detecting a peak can be precisely predicted based on asignal that is given.

For example, the level of a sine wave becomes highest at 90 degrees, andthe level of a rectangular wave (with no DC) that has passed through afirst-order integration circuit shows highest positive value or negativevalue just before the polarity of the wave changes.

From what has been described above, in detecting a liquid, use of analternating current is advantageous than use of a direct current, sothat an alternating current is used in the present invention.

Now, liquid-amount detecting apparatuses according to embodiments of thepresent invention will be described.

First Embodiment

FIG. 4 is a diagram showing the construction of a liquid-amountdetecting apparatus 10 according to a first embodiment of the presentinvention. As shown in FIG. 4, a conductive liquid that is to bedetected by the liquid-amount detecting apparatus 10 is contained incontainers T (T1 and T2).

For example, if the liquid-amount detecting apparatus 10 is used in anink-jet printer, the containers T are ink tanks, and the liquid in thecontainers T is ink that is used in the ink-jet printer. In the case ofa color ink-jet printer that uses ink of a plurality of colors, acontainer T (ink tank) is provided for each of the colors.

The liquid-amount detecting apparatus 10 according to this embodimentincludes a liquid detecting circuit 20, a controller 30, and aremaining-amount indication unit 40.

The liquid detecting circuit 20 includes an AC-signal source (V1) 21, animpedance (Zs) 22, a switch (SW) 23, a threshold detecting unit 24, adata extracting unit 25, and a detector substrate 27 having electrodeunits 26 (26 a to 26 e). A specific circuit configuration of the liquiddetecting circuit 20 will be described later.

An AC signal generated by the AC-signal source 21 passes through theimpedance 22 (The impedance 22 is a source impedance to form anattenuator with the contact resistance.), whereby a DC component thereofis removed, and the resulting AC signal not containing the DC componentis fed to the electrode units 26. A sufficient potential difference isgenerated depending on whether the electrode units 26 are in contactwith the liquid.

The switch 23 is controlled so that the AC signal fed from the ACsignal-source 21 via the impedance 22 will be input to a selected one ofthe electrode units 26.

The electrode units 26 are formed by paired electrodes 26 a to 26 e thatare disposed so as to be in contact at least partially with the liquidin the container T. When in contact with the liquid, the pairedelectrodes 26 a to 26 e becomes electrically conductive. In thisembodiment, the electrode units 26 are provided on the detectorsubstrate 27, and the detector substrate 27 is disposed inside thecontainer T.

In this embodiment, the electrode units 26 are disposed inside thecontainer T, and parts of the liquid-amount detecting apparatus 10 otherthan the electrode units 26 are disposed outside the container T.

In this embodiment, four pairs of electrodes (26 a to 26 d with 26 e)are provided in one container T (each electrode pair is enclosed in adotted ellipse in FIG. 4). The electrode pairs are formed by detectingelectrodes 26 a to 26 d and a common electrode 26 e. The detectingelectrode 26 a and the common electrode 26 e, the detecting electrode 26b and the common electrode 26 e, the detecting electrode 26 c and thecommon electrode 26 e, and the detecting electrode 26 d and the commonelectrode 26 e are disposed in proximity to each other, formingelectrode pairs.

The detecting electrodes 26 a to 26 d are disposed in parallel at aregular interval in the vertical direction. When the liquid in thecontainer T is decreased, the surface of the liquid shifts from upper tolower as viewed in FIG. 4. That is, the liquid surface becomes lower inthe vertical direction when the amount of the liquid decreases.

The detecting electrode 26 a is disposed at an uppermost position amongthe detecting electrodes 26 a to 26 d. This is a position that comesinto contact with the liquid in the container T when the container T isfull. The detecting electrode 26 is disposed in the proximity of thebottom surface of the container T.

Furthermore, one common electrode 26 e is provided on one detectorsubstrate 27, and the single common electrode 26 e is associated withall the four detecting electrodes 26 a to 26 d. The common electrode 26e is connected to the ground (GND). (The common electrode 26 e should beconnected to a common connection with a certain potential or a ground.But, grounding is not needed as long as a flow of a direct current isprevented; however, the common electrode 26 e is grounded since theground is usually used as a reference of potential at the thresholddetecting unit 24.)

All the detecting electrodes 26 a to 26 d and the common electrode 26 eare formed so as to have substantially the same surface area, shape,etc., so that their impedance characteristics can be nearly equal. Thisis because if impedance characteristics differ among the electrode units26, a range for detecting correct status of liquid becomes narrower(Detections are made by a single circuit.).

Although two containers T1 and T2 are shown in FIG. 4, the number ofcontainers T is arbitrary. When more containers T are provided, thedetector substrate 27 described earlier is provided for each additionalcontainer T, and put additional nodes 23 a of the switch 23, associatedwith the detecting electrodes 26 a to 26 d for the container T.Furthermore, the common electrode 26 e for the container T added isconnected to a line to which the common electrodes 26 e for thecontainers T1 and T2 are connected, and is thereby grounded.

The threshold detecting unit 24 outputs a signal representing the statusof electric connection between each of the pairs of electrodes 26 a to26 d and 26 e.

The data extracting unit 25 outputs a binary signal representing thepresence or absence of electric connection between each of the pairs ofelectrodes 26 a to 26 d and 26 e, based on the signal output from thethreshold detecting unit 24.

The controller 30 has a CPU and a memory (storage device), and itincludes a determining unit 31 for determining the presence or absenceof the liquid at the electrode units 26 based on the binary signaloutput from the liquid detecting circuit 20. Furthermore, the controller30 is capable of controlling switching of the nodes 23 a of the switch23 (node-select function).

The remaining-amount indication unit 40 displays the remaining amount ofthe liquid in the container T in steps, based on the result ofdetermination by the determining unit 31 of the controller 30. In thisembodiment, the remaining amount is represented in five steps.

An AC signal output from the AC-signal source 21 passes through theimpedance 22, whereby a DC component in the AC signal is removed. Theresulting AC signal is fed to the switch 23.

The switch 23 electrically connects the AC-signal source 21 to one ofthe detecting electrodes 26 a to 26 d. That is, the switch 23 forwardsthe AC signal having passed through the impedance 22 to a selected oneof the detecting electrodes 26 a to 26 d.

When the liquid is present between each of the pairs of electrodes 26 ato 26 d and 26 e, the detecting electrodes 26 a to 26 d are electricallyconnected to the common electrode 26 e. Thus, a current flows betweenthe detecting electrodes 26 a to 26 d and the common electrode 26 e, andis forwarded to the ground. Accordingly, the signal input to thethreshold detecting unit 24 exhibits no significant change in voltage(Since the signal from V1 is sufficiently attenuated.). On the otherhand, when the liquid is not present between each of the pairs ofelectrodes 26 a to 26 d and 26 e, the detecting electrodes 26 a to 26 dare virtually open circuited. Thus, no significant current flows betweenthe detecting electrodes 26 a to 26 d and the common electrode 26 e.Accordingly, the signal input to the threshold detecting unit 24exhibits nearly the same level as that of V1:21.

When the signal described above is input to the threshold detecting unit24, a threshold detection is performed, and an output value of thedetection is input to the data extracting unit 25. The data extractingunit 25 carries out a synchronous detection. The data extracting unit 25receives a clock signal for detection from the AC-signal source 21, theclock signal being controlled so as to be synchronized with the signalinput from the threshold detecting unit 24. The clock signal and the ACsignal are originally the same signal generated by the AC-signal source21 with this embodiment, so that the cycles of these signals can besynchronized with each other. Since the signals are synchronous witheach other, measurement can be speeded up by a use of the synchronousdetection. Obviously, the clock signal may be generated separately fromthe AC signal by another signal source. In that case, synchronousdetection is facilitated by synchronizing the two signals, achieving thesame effect as in the case where the clock signal and the AC signal arethe same signal generated by the same signal source.

The data extracting unit 25 outputs a binary signal representing thepresence or absence of electric connection between each of the pairs ofelectrodes 26 a to 26 d and 26 e. The determining unit 31 receives thebinary signals, and determines the presence or absence of the liquid atthe electrode units 26 based on a combination of the binary signals.

Furthermore, a signal representing the result of determination by thedetermining unit 31 is input to the remaining-amount indication unit 40.The remaining-amount indication unit 40 includes, for example, adisplay, which displays the remaining amount of the liquid in eachcontainer T in five steps. For example, if the liquid is detected by allthe four electrode units 26 as to the remaining amount of liquid in onecontainer T, “4” is displayed. If the liquid is detected by the lowerthree electrode units 26 but not by the uppermost electrode unit 26 (thedetecting electrode 26 a and the common electrode 26 e), “3” isdisplayed. Similarly, if the liquid is detected by none of the fourelectrode units 26, “0” is displayed.

FIG. 5 is a waveform chart for explaining a detecting operation in thisembodiment. The detecting operation shown in FIG. 5 is only an examplefor illustrating the detecting operation, and not related to the statusof the electrode units 26 and the amount of ink in the containers Tshown in FIG. 4. That is, for the purpose of explanation, the detectingoperation shown in FIG. 5 is an example where status changes as “inkpresent”, “ink absent”, “ink present”, and “ink absent”.

In FIG. 5, part (A) is a waveform chart of an AC signal output from theAC-signal source 21. The AC signal is a rectangular wave having a periodof 2 μsec and an amplitude of +5.0 V to 0 V.

Part (B) is a waveform chart of the AC signal fed from the AC-signalsource via the impedance 22, with a DC component having been removedtherefrom, and the amplitude thereof ranges from +2.5 V to −2.5 V.

As indicated by P1 in part (B), the connection between the switch 23 andthe nodes 23 a is switched at a cycle of the period of the AC signal (2μsec). More specifically, the connection between the switch 23 and thenodes 23 a is switched at the timing of a falling edge of therectangular wave, indicated by an arrow P2.

Thus, during the first period of the AC signal (0 to 2 μsec), the switch23 is connected to a node 23 a associated with the detecting electrode26 a for the container T1. Then, at the timing of 2 μsec, the switch 23is switched to a node 23 a associated with the detecting electrode 26 bfor the container T1. Thus, in the second period (2 to 4 μsec), the ACsignal from the AC-signal source 21 is fed to the detecting electrode 26b. Thus, connection with the electrode units 26 can be efficientlyswitched by controlling switching of the switch 23 in synchronizationwith the AC signal from the AC-signal source 21.

Furthermore, without limitation to detection of the liquid in a singlecontainer T, by switching connection with the electrode units 26sequentially for all the containers T1, T2, . . . , the single liquiddetecting circuit 20 can be connected, by time division, to theelectrode units 26 of all the containers T.

In FIG. 5, part (C) is a waveform chart showing signals input from theelectrode units 26 to the threshold detecting unit 24. The waveform inthe first period of 0 to 2 μsec represents the status of electricconnection between the detecting electrode 26 a and the common electrode26 e for the container T1. The waveform in the next period of 2 to 4μsec represents the status of electric connection between the detectingelectrode 26 b and the common electrode 26 e for the container T.

The signals input from the electrode units 26 are input to the thresholddetecting unit 24, where a threshold detection is performed.

Part (D) is a waveform chart showing a signal output from the thresholddetecting unit 24. In this example, a threshold P3 (substantially −1 Vin this example) is set in the negative side to output the status ofsignal attenuation at the electrode units 26. That is, when a valueinput from the electrode units 26, which is in a range of +2.5 V to −2.5V, becomes more negative than the threshold value P3, as enclosed in adotted ellipse in part (C), an inverted value is output.

Part (E) is a waveform chart showing a signal output from the dataextracting unit 25. Referring to the waveform chart in part (D),synchronous detection is performed based on the cycle of the clocksignal, and a binary signal representing whether the voltage isapproximately +5 V is output. In the example shown in FIG. 5, detectionis performed at the timings of 1, 3, 5, . . . μsec, as indicated by anarrow P4. For example, detection is performed at 1 μsec in the firstperiod from 0 to 2 μsec. In the waveform chart shown in part (D), thesignal has a voltage of approximately +5 V, so that a signalrepresenting “voltage present” is output. This signal is maintaineduntil the next detection.

The next detection is performed at 3 μsec. At this time, the signal inthe waveform chart shown in part (D) does not have a voltage ofapproximately +5 V, so that a signal representing “voltage absent” isoutput. Similarly to the above case, this signal is maintained until 5μsec, which is the timing of the next detection. By performing detectionin synchronization with the clock signal as described above, (every)detection can be performed at a stable timing.

Next, a specific circuit configuration of the liquid detecting circuit20 will be described. FIG. 6 is a circuit diagram of the liquiddetecting circuit 20 according to this embodiment.

An AC-signal source V1 (21) in this embodiment uses a signal having anamplitude of 0 to 5 V and a frequency of 250 kHz, which is used in aCMOS logic circuit.

A capacitor C1 removes the DC component in an AC signal fed from theAC-signal source V1. The capacitor C1 is grounded via a resistor R1having a resistance of 4.7 kΩ. Furthermore, the capacitor C1 isconnected to the switch 23 via a resistor R4 having a resistance of 22kΩ. In this circuit diagram, an impedance network Zs is implemented by aT-shaped circuit formed by the capacitor C1 and the resistors R1 and R4.

Transistors Q1 and Q2, in combination with transistors Q3 and Q4, formdifferential amplifiers, which compare a signal detected by thedetecting electrodes 26 a to 26 d connected to the base of thetransistor Q3 with a threshold (nearly −1 V with this embodiment) presetat the base of the transistor Q4, thereby detecting change in thesignal.

Furthermore, the arrangement is such that a current flows to thecollector of the transistor Q4 only when the base potential of thetransistor Q3 has dropped below that of the transistor Q4. Practically,a current flows only when the signal applied for detection is on thenegative side and is lower than the threshold value (i.e., when theliquid is not in contact with the electrode units 26).

Transistors Q5 and Q6 cause a collector current of the transistor Q4 tobe inverted and the inverted current to flow to the collector of thetransistor Q6, whereby a voltage is generated across a resistor R5having a resistance of 3.3 kΩ. A voltage is generated across theresistor R5 only when it is determined that the electrode units 26 arenot in contact with the liquid.

With regard to the voltage generated across the resistor R5, therelationship between the collector current of the transistor Q6 and theresistor R5 is chosen so that the transistor Q6 can be saturated (at amaximum potential of approximately 5 V). Thus, when a voltage isgenerated across the resistor R5, a signal that is sufficient fordetection by a DFF (D flip-flop) of a CMOS that performs nextsynchronous detection is fed to a D input terminal thereof.

The DFF receives, via a CLK input terminal thereof, a clock signal(detection signal) that is exactly the same as the AC signal describedearlier, and carries out determination.

In FIG. 6, the AC signal output from the AC-signal source V1 and theinput to the capacitor C1 and the clock signal input to the CLK inputterminal of the DFF correspond to the signal represented by the waveformchart shown in part (A) of FIG. 5. The AC signal having passed throughthe capacitor C1, which does not include the DC component, correspond tothe signal represented by the waveform chart shown in part (B) of FIG.5.

The signal input from the electrode units 26 (Detector-Input)corresponds to the signal represented by the waveform chart shown inpart (C) of FIG. 5. The signal fed to the D input terminal of the DFF(Detector-Output) corresponds to the signal represented by the waveformchart shown in part (D) of FIG. 5. The signal output from the DFF(Phase-Detector-Output) corresponds to the signal represented by thewaveform chart shown in part (E) of FIG. 5.

Second Embodiment

FIG. 7 is a waveform chart showing a second embodiment of the presentinvention, and it corresponds to FIG. 5 for the first embodiment.

In the first embodiment, the AC signal with a DC component having beenremoved therefrom is a rectangular wave; whereas in the secondembodiment, a sine wave is used.

In FIG. 7, an original signal output from the AC-signal source 21 is arectangular wave, as shown in part (A). This signal is converted, forexample, through a low-pass filter, into a sine (or like) wave shown inpart (B).

The signal shown in part (B) is obtained by converting a rectangularwave into a sine (or like) wave and removing a DC component from thesine wave. In part (A) of FIG. 7, as compared with part (A) of FIG. 5,the phase is shifted by ¼, as indicated by P5. Thus, the sine wavecrosses 0 V at 1, 2, 3, . . . μsec.

Then, detection is performed when the clock signal rises (when the sinewave reaches a minimum level). The timing of detection is indicated byan arrow P4, as in FIG. 5.

In the case where a sine wave is used, as compared with the case where arectangular wave is used, advantageously, the signal requires a narrowerbandwidth, and therefore, the problem of undesired radiation can bealleviated. Furthermore, since the waveform is not considerably affectedby the environment and conditions (because of the nature of the sinewave) during detection, detection can be performed appropriately even ina large-sized apparatus in which the distance to a detection point tendto be long. Furthermore, detection speed can be improved compared withthe case where a rectangular wave is used because of a use of a higherclock frequency. However, as described earlier, the sine wave must besynchronized with the system.

It is also possible to use a low-pass filtered rectangular wave. In thatcase, the impedance (Zs) 22 is implemented by a low-pass filter and aresistor for adjusting impedance.

Third Embodiment

FIG. 8 is a circuit diagram showing a third embodiment of the presentinvention, and it corresponds to FIG. 6 for the first embodiment. FIG. 9is a waveform chart relating to the circuit diagram shown in FIG. 8, andit corresponds to FIG. 5 for the first embodiment.

In the first embodiment, power sources of ±5 V are required, as shown inFIG. 6. In contrast, in the third embodiment, only a power source V2 of+5 V suffices to achieve the same functions as the first embodiment.

In this circuit, the average voltage of measurement equals the DCcomponent of the clock signal. Thus, if a 5 V power source is used,measurement is performed at 2.5 V or nearly as a center. For the purposeof comparison, a DC power source V3 of 2.2 V, connected to the base ofthe transistor Q2, is used.

Furthermore, although all the nodes 23 a of the switch 23 are connectedto the detecting electrodes 26 a to 26 d in the first embodiment, a node23 a′ that is connected to the common electrode 26 e and is therebygrounded is additionally provided in the third embodiment.

For example, when the power of the liquid detecting apparatus 10 is off,the switch 23 is selectively connected to the node 23 a′.

For example, when the liquid detecting apparatus 10 is powered on oroff, the node 23 a′ is selected, whereby the capacitor C1 is quicklycharged or discharged without causing a current to flow throughelectrode units 26 that are in contact with the liquid. That is,immediately after the liquid detecting apparatus 10 is powered on orwhen the liquid detecting apparatus 10 is not in operation, a potentialdifference remains between each of the pairs of electrodes (between thedetecting electrodes 26 a to 26 d and the common electrode 26 e). Theremaining potential difference is reduced as time elapses. However, ifthis is repeated many times, the electrolysis of the liquid couldprogress. Thus, in the third embodiment, in order to avoid thissituation, the switch 23 is connected to the node 23 a′ while the systemis not ready for measurement by the liquid detecting apparatus 10.

Fourth Embodiment

FIG. 10 is a circuit diagram showing a fourth embodiment of the presentinvention, and it corresponds to FIG. 6 for the first embodiment. FIG.11 is a waveform chart relating to the circuit shown in FIG. 10, and itcorresponds to FIG. 5 for the first embodiment.

The circuit according to the fourth embodiment, similarly to the thirdembodiment, uses a single power source V2 of +5 V. In the thirdembodiment, the DC power source V3 of 2.2 V is used for comparison anddetection. In contrast, in the fourth embodiment, a clock signal havingpassed through resistors R2 and R5, with a DC component maintained as itis, is applied equally to the bases of the transistors Q1 and Q2 servingas inputs to the threshold detecting unit 24, and a threshold detectionis carried out at approximately one half of the power-supply voltage,i.e., at 2.5 V.

Furthermore, the base potential of the transistor Q1 must be maintainedhigher by the threshold value, that is, in this embodiment, detectionmust be performed using signals on one side of an intermediate level.Thus, the resistor R4 having a resistance of 220 kΩ is used for a slightvoltage division, whereby the base potential of the transistor Q2 islowered.

With the circuit configuration described above, stable detection isallowed even if the power-supply voltage fluctuates. Furthermore, thesignal voltage remaining at the base of the transistor Q1 when theelectrode units 26 are in contact with the liquid can be virtually equalto the signal voltage that is applied to the base of the transistor Q2,so that the output is hardly affected. That is, the S/N ratio ofdetection can be improved (the dynamic range can be increased).

In the waveform chart shown in FIG. 11, as will be understood from part(B) (Vb(Q1)−Vb(Q2)), the clock signal V (Detector-Input) that appearsimmediately below, which attenuates in accordance with the ratio of theresistance of the signal-source resistor R2 (20 kΩ) and the conductionresistance of the liquid as measured with an alternating current(assumed to be 500 Ω in this embodiment), can be virtually cancelled, asindicated by P6 enclosed in a dotted ellipse in part (B).

In principle, the value of the attenuated clock signal can be virtuallycancelled if the total of the resistance of the switch 23 and theconduction resistance of the electrode units 26 in the liquid is equalto the resistance of the resistor R1 (820 Ω in this embodiment). Thus,it is possible to use a variable resistor for the resistor R1 to allowadjustment in accordance with an actual state.

The embodiments described above exhibits the following advantages:

(1) Since a complete cycle or several complete cycles of an AC currentflows through the detecting electrodes 26 a to 26 d, liquid is preventedfrom ionization, serving to avoid change in liquid characteristics.

(2) Electric signals are processed separately from the containers T, andno DC current flows and only a weak AC current flows. Thus, the safetyof a system that deals with aqueous liquid can be improved.

(3) Measurements at the individual electrode units 26 determine thepresence or absence of electric connection instead of determining analogamounts. Thus, no adjustment is needed, reliability is improved, and theprecision of measurement is determined only by the number of electrodeunits 26 provided.

(4) With the electrode units 26 provided for measurement, only oneliquid detecting circuit 20 suffices. Thus, the liquid-amount detectingapparatus 10 can be implemented simply and inexpensively.

(5) Since conduction resistance is lower compared with DC detection, thearea of the electrode units 26 can be made small. Thus, precisedetection is allowed without occupying a large space, and a large numberof electrode units 26 can be disposed.

(6) Since measurement can be performed quickly compared with DCdetection, the speed of measurement and display as a whole can beimproved.

(7) Since power consumption is small, even a battery-powered operationis possible.

(8) Since signals in the audio to the AM frequency band can be used,substantially no particular measure is required against undesiredradiation.

(9) Since the operation of the single liquid detecting circuit 20suffices constantly for all the detecting electrodes 26 a to 26 d, theeffect of mutual cross-talks during observation and detection can besubstantially eliminated.

(10) Since a container T has to contain only the electrode units 26, thestructure of the container T can be simplified.

Although the present invention has been described in the context ofspecific embodiments, the present invention is not limited to theembodiments described herein, and various modifications are possible,including the following:

(1) The liquid-amount detecting apparatuses 10 according to theembodiments can be used in various apparatuses for detecting and/ordisplaying the presence or absence of various liquids or the remainingamount of thereof in various containers T, without limitation todetecting the remaining amount of ink in an ink-jet printer.

(2) In the embodiments described above, the remaining amount of liquidin a container T is represented by values of “0” to “4”. Alternatively,four LEDs may be provided for each container T, indicating the remainingamount of liquid by turning the LEDs on or off. For example, when theliquid is detected by all the four electrode units 26, all the LEDs areturned on. If the liquid is detected by the lower three electrode units26 but not by the uppermost electrode unit 26 (the detecting electrode26 a and the common electrode 26 e), three LEDs are turned on and oneLED is turned off. If the liquid is detected by none of the LEDs, allthe LEDs are turned off.

(3) In the third and fourth embodiments, the node 23 a′ that isconnected to the ground is provided as one of nodes of the switch 23.Alternatively, for example, the arrangement may be such that the switch23 can be disconnected from all the nodes. That is, the arrangement maybe arbitrary as long as the detecting electrodes 26 a to 26 d can beelectrically disconnected.

(4) In the embodiments, the presence or absence of liquid is detected byall the electrode units 26. Alternatively, for example, detection may besequentially performed in a single container T starting from theuppermost electrode unit 26 (the detecting electrode 26 a and the commonelectrode 26 e), skipping detection of the presence or absence of theliquid by electrode units 26 below an electrode unit 26 with which thepresence of the liquid has been detected.

Furthermore, without limitation to detection of the presence or absenceof liquid or the remaining amount of liquid in a container T, liquid inother parts can also be detected. For example, when the apparatus isused in an ink-jet printer, electrode units 26 may be provided in achamber (ink pool) disposed at the immediate upstream of a printer head,detecting the presence or absence of ink in the chamber. Furthermore, inorder to protect the printer head, it is possible to exercise control soas to stop printing if it is determined that ink is not present in thechamber.

(5) The impedance 22 for removing a DC component in a signal fed fromthe AC-signal source 21 can be implemented by various elements, forexample, one or more capacitors or resistors, or a combination thereof.If the original signal generated by the AC-signal source 21 does notinclude a DC component, the impedance 22 can be implemented only by aresistor. If a DC component needs to be removed, a capacitor isconnected in series with a resistor.

(6) In the embodiments, a plurality of electrode units 26 is provided ina single container T to detect the remaining amount of liquid in thecontainer T. Alternatively, for example, a single electrode unit 26 maybe provided at the bottom of the container T to detect only the presenceor absence of liquid.

According to the present invention, since a direct current does not flowthrough liquid, the characteristics of the liquid do not change.Furthermore, conduction resistance can be made small. Furthermore,detection speed can be increased.

Furthermore, since the presence or absence of liquid is determined byoutputting a binary signal, digital processing is allowed, serving toimprove the reliability of detection.

1. A liquid holding tank for a printer comprising: at least onecontainer which contains a liquid that is used in a printer; and aliquid detecting apparatus; and wherein at least a portion of saidliquid detecting apparatus is disposed at said container; said liquiddetecting apparatus comprising: a liquid detecting circuit comprising anelectrode unit formed by a pair of electrodes that is to be disposed incontact at least partially with the liquid in the container, the pair ofelectrodes being electrically connected to each other when the pair ofelectrodes is in contact with the liquid; a source impedance; and analternating-current signal source; wherein the liquid detecting circuitinputs an alternating-current signal not containing a direct-currentcomponent to the electrode unit through the source impedance, andoutputs a binary signal representing the presence or absence of anelectrical connection between the pair of electrodes based on the outputsignal; and a determining unit for determining the presence or absenceof the liquid at the electrode unit based on the binary signal outputfrom the liquid detecting circuit and wherein an electrode unit isprovided in each of a plurality of containers, and wherein the liquiddetecting circuit sequentially outputs, a status of electricalconnection between each of the respective pairs of electrodes in theplurality of containers.
 2. A liquid holding tank according to claim 1,wherein the binary signal representing the presence or absence of anelectrical connection between the pair of electrodes is output based ona period of the alternating signal generated by the alternating-currentsignal source.
 3. A liquid holding tank according to claim 1, whereinthe liquid detecting circuit generates a clock signal for outputting thebinary signal representing the presence or absence of electricalconnection between the pair of electrodes, and exercises control so thatthe alternating-current signal generated by the alternating-currentsignal source and the clock signal are synchronized with each other. 4.A liquid holding tank according to claim 1, wherein a plurality of theelectrode units are provided in the single container and disposed at aplurality of positions in a direction of lowering of a liquid surface inaccordance with decrease of the liquid in the container, wherein theliquid detecting circuit outputs binary signals representing thepresence or absence of electrical connection between the respectivepairs of electrodes, and wherein the determining means determines, in astepwise manner, the remaining amount of the liquid in the containerbased on the binary signals associated with the respective pairs ofelectrodes.
 5. A liquid holding tank according to claim 1, wherein aplurality of the electrode units are provided in the single containerand disposed at a plurality of positions in a direction of lowering of aliquid surface in accordance with decrease of the liquid in thecontainer, wherein the liquid detecting circuit outputs, based on aperiod of the alternating-current signal generated by thealternating-current signal source, binary signals representing thepresence or absence of electrical connection between each respectivepair of electrodes, wherein the determining means determines, in astepwise manner, the remaining amount of the liquid in the containerbased on the binary signals associated with each respective pair ofelectrodes, and wherein the alternating-current signal source isconnected to the plurality of electrode units such that a connection ofthe alternating-current signal source with one of the plurality ofelectrode units can be switched to a connection with another one of theplurality of electrode units, the connection between thealternating-current signal source and each of the plurality of electrodeunits being switched in synchronization with the period of thealternating-current signal.
 6. A liquid holding tank according to claim1, wherein a plurality of the electrode units are provided in the singlecontainer and disposed at a plurality of positions in a direction oflowering of a liquid surface in accordance with decrease of the liquidin the container, wherein the liquid detecting circuit outputs binarysignals representing the presence or absence of electrical connectionbetween the respective pairs of electrodes, wherein the determiningmeans determines, in a stepwise manner, the remaining amount of theliquid in the container based on the binary signals associated with therespective pairs of electrodes, wherein the liquid detecting circuitgenerates a clock signal for outputting the binary signals representingthe presence or absence of electrical connection between the respectivepairs of electrodes, and exercises control so that thealternating-current signal generated by the alternating-current signalsource and the clock signal will be synchronized with each other, andwherein the alternating-current signal source is connected to theplurality of electrode units such that a connection of thealternating-current signal source with one of the plurality of electrodeunits can be switched to a connection with another one of the pluralityof electrode units, the connection between the alternating-currentsignal source and each of the plurality of electrode units beingswitched in synchronization with the clock signal.
 7. A liquid holdingtank according to claim 1, wherein the electrode unit of the liquiddetecting circuit is disposed inside the container, and wherein parts ofthe liquid detecting circuit other than the electrode unit and thedetermining means are disposed outside the container.
 8. A liquidholding tank according to claim 1, wherein a plurality of the electrodeunits are provided, and wherein-impedance characteristics of theplurality of electrode units are common so as to facilitatedetermination for binarization.
 9. A liquid holding tank according toclaim 1, wherein the liquid detecting circuit outputs the binary signalrepresenting the presence or absence of electrical connection betweenthe pair of electrodes using at least one of a positive-polarity signaland a negative-polarity signal of the alternating-current signal.
 10. Aliquid holding tank according to claim 1, wherein the liquid detectingcircuit is allowed to disconnect the alternating-current signal sourcefrom a node of the electrode unit to which the alternating-currentsignal is input.
 11. A liquid holding tank according to claim 1, whereinthe liquid detecting circuit is allowed to disconnect thealternating-current signal source from a node of the electrode unit towhich the alternating-current signal is input, and to thereby connectthe alternating-current signal source to a grounding point or aconnection at a predetermined potential.
 12. A liquid-amount detectingmethod for detecting an amount of a liquid contained in a containerwhich contains a liquid that is used for a printer, inputting analternating-current signal not containing a direct-current componentfrom an alternating-current signal source of a liquid detecting circuitto an electrode unit through a source impedance, the electrode unitbeing formed by a pair of electrodes that is to be disposed in contactat least partially with the liquid in the container, the pair ofelectrodes being electrically connected to each other when the pair ofelectrodes is in contact with the liquid, wherein a binary signalrepresenting the presence or absence of electrical connection betweenthe pair of electrodes is output, and wherein the presence or absence ofthe liquid at the electrode unit is determined based on the binarysignal, and wherein the electrode unit is provided in each of aplurality of containers, and wherein the liquid detecting circuitsequentially outputs, a status of electrical connection between each ofthe respective pairs of electrodes in the plurality of containers.
 13. Aliquid amount detecting method according to claim 12, wherein the binarysignal representing the presence or absence of electrical connectionbetween the pair of electrodes is output based on a period of thealternating signal generated by the alternating-current signal source.14. A liquid amount detecting method according to claim 12, wherein aclock signal for outputting the binary signal representing the presenceor absence of electrical connection between the pair of electrodes isgenerated, and wherein control is exercised so that thealternating-current signal generated by the alternating-current signalsource and the clock signal will be synchronized.
 15. A liquid amountdetecting method according to claim 12, wherein a plurality of theelectrode units are provided in the single container and disposed at aplurality of positions in a direction of lowering of a liquid surface inaccordance with decrease of the liquid in the container, wherein binarysignals representing the presence or absence of electrical connectionbetween the respective pairs of electrodes are output, and wherein theremaining amount of the liquid in the container is determined in astepwise manner based on the binary signals associated with therespective pairs of electrodes.